1. Field of the Invention
This invention relates generally to a system and method for decoding a received transmission and, more particularly, to a system and method for storing and processing a received packet of symbols to reduce packet error rate (PER) for an orthogonal frequency division multiplexing (OFDM) protocol for a vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) dedicated short range communications (DSRC) system.
2. Discussion of the Related Art
As vehicles have become more and more technologically advanced, a need has arisen for a reliable vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications system, such as a dedicated short range communications (DSRC) system. By using vehicular networks of this type, vehicles can share traffic flow information, alert other vehicles of hazardous road conditions, help drivers be more aware of neighboring vehicles, etc. In addition, reliable vehicular communications is essential to aid in the operation of autonomously driven vehicles.
In digital communications systems of the type referred to above, the digital data being transmitted are modulated onto a carrier wave to include information symbols that are represented by orthogonal sinusoids that identify the data in the transmitted messages. The various components in the transmitter, the transmission medium and the receiver cause various types of distortions in the transmitted signal. These distortions include signal dispersion or smearing that causes pulses in the received signal to not be well defined. If the distortion is not corrected in the receiver during the demodulation process, data can be lost, resulting in unreliable transmissions. Therefore, processes, known as channel equalization, are performed in the receiver on the received signal to remove the distortions and correct for the effects of the channel.
The IEEE 802.11p communications standard is currently the core communication protocol for vehicular networks. The 802.11 communications standard employs a protocol stack including various layers, such as a physical layer, a medium access control (MAC) layer, etc., that each perform different operations, as is well understood by those skilled in the art. This communications standard encodes the digital data that is to be transmitted in the transmitter and deciphers decoded data when it is received at the receiver. For the physical layer (PHY), the IEEE 802.11p standard uses orthogonal frequency division multiplexing (OFDM), where OFDM is a spectrally efficient multi-carrier modulation scheme. OFDM separates the usable bandwidth, typically 10-20 MHz, into 52 orthogonal sub-channels or subcarriers at different frequencies. Of those 52 subcarrier frequencies, 48 subcarrier frequencies are used for data transmission and four subcarrier frequencies are used for pilot transmission. The pilots are used for center frequency offset tracking, as is well understood by those skilled in the art.
The subcarriers within an OFDM signal are orthogonal to each other in both the time and frequency domains, such that they do not interfere with each other. OFDM employs a cyclic prefix, also known as a guard interval, at the beginning of each symbol. This cyclic prefix maintains subcarrier orthogonality and is used to prevent inter-symbol interference. The cyclic prefix thus helps protect OFDM from multipath effects.
The 802.11p PHY is similar to the 802.11a PHY with two primary differences, namely, the 802.11p standard uses a 10 MHz bandwidth, where the 802.11a standard uses 20 MHz, and the 802.11p standard uses an operating frequency of 5.9 GHz, where the 802.11a standard uses an operating frequency of 5 GHz. When using a binary phase-shift keying (BPSK) modulation scheme with 1/2 coding rate, this yields a data rate of 3 Mb/s.
All of the above described features make the 802.11p standard a good choice for a high data rate communications protocol for an outdoor channel. However, performance of the 802.11p standard over V2V channels is far from optimal. In previous work, the statistical characteristics of the V2V channel were measured, and the feasibility of using different time scaled OFDM waveforms was studied. The primary detriment to performance of the 802.11p standard is the channel's short coherence time. Particularly, because the 802.11p standard does not restrict the length of message packets, a short coherence time is a major concern. Short packets will naturally have better performance, whereas longer packets will suffer as a result of the short coherence time of the channel. Therefore, enhanced channel equalization for the 802.11p standard is needed in an effort to reduce packet error rate (PER). Improving the performance at the physical layer will result in improved performance at all layers.
The IEEE 802.11p PHY is based on the PHY found in the 802.11a standard. This standard was designed for indoor use, and as such performs well for indoor environments. However, outdoor environments feature a more dynamic channel and a longer delay spread in the signal. This leads to the guard interval in the 802.11a standard being too short for outdoor use. Several methods for addressing an excessively long delay spread in the 802.11p standard are known. However, the 802.11p standard has a guard interval that is twice as long as the guard interval in the 802.11a standard. Based on channel measurements, the long delay spread is not a significant problem affecting the 802.11p standard, and therefore, the short coherence time is more important.
Several techniques exist to improve the performance and accuracy of the initial channel estimate in packetized message transmissions. While this type of technology is important for the 802.11a standard, the short coherence time of a V2V channel nullifies any gains realized by a more accurate initial estimation.
For packetized OFDM transmissions, tracking the channel is important. Some work has been done on adaptive channel tracking algorithms for a DSRC system and/or the 802.11 standard. Decision directed channel feedback from data symbols is determined by decoding and demodulating symbols to re-estimate the channel throughout the packet. This method is more complex in that it requires Viterbi decoding and remodulation of OFDM symbols as equalization is taking place. A similar technique has been employed for 802.11a packets that are applied to a vehicular environment. This works in tandem with a time domain equalizer that helps to reduce the effects of multipath and inter-symbol interference. An adaptive technique using vehicle speed, signal-to-noise ratio and packet length has been proposed to aid in tracking the channel using data symbols. A least mean squares (LMS) algorithm has been used in conjunction with pilot data to correct for residual carrier frequency offset and channel conditions throughout the length of the packet.